Synthesis and Characterization of the Layered Zirconium Arsenate Zr2O3(HAsO4)·nH2O

Materials Research Bulletin, Vol. 33, No. 4, pp. 583–590, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/98 $19.00 1 .00

PII S0025-5408(98)00013-0

SYNTHESIS AND CHARACTERIZATION OF THE LAYERED ZIRCONIUM ARSENATE Zr2O3(HAsO4)znH2O

1

A.I. Bortun1, L.N. Bortun1, A. Clearfield1*, C. Trobajo2, and J.R. Garcı´a2* Texas A&M University, Chemistry Department, College Station, TX 77843-3255, USA 2 Departamento de Quı´mica Orga´nica e Inorga´nica, Universidad de Oviedo, 33071 Oviedo, Spain (Refereed) (Received August 5, 1997; Accepted August 9, 1997)

ABSTRACT Layered sodium zirconium arsenate of composition Zr2O3(NaAsO4)z3H2O was prepared by the reaction between Zr(OC3H7)4 and sodium arsenate in alkaline media (pH . 12) under mild hydrothermal conditions (180–200°C). Two hydrogen forms of the zirconium arsenate (c-ZrAs) Zr2O3(HAsO4)z3H2O and Zr2O3(HAsO4)zH2O, were prepared by acid treatment of the sodium form. The intercalation of n-alkylamines into the c-ZrAs from the gas phase was studied. The synthesized materials were characterized by elemental analysis, thermogravimetric analysis, infrared spectroscopy and powder X-ray diffraction. The data indicate that the zirconium arsenate is isostructural to c-Zr2O3(HPO4)znH2O (n 5 0.5, 1.5). The new compounds exhibit high hydrolytic stability in alkaline media. The ion exchange behavior of the c-Zr2O3(HAsO4)z3H2O towards alkali, alkaline-earth, and some di- and tri-valent metal cations in different solutions was studied over a wide pH range (2–14) by the batch technique. © 1998 Elsevier Science Ltd KEYWORDS: A. layered compounds, A. oxides, B. intercalation reactions, C. infrared spectroscopy, C. X-ray diffraction INTRODUCTION Among the various inorganic materials of current interest, layered polyvalent metal phosphates occupy a special position. This attention is related to the possibility of their application

*Authors to whom correspondence should be addressed. 583

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as highly selective ion exchangers, acid catalysts, protonic conductors, and novel functionalized materials [1– 4]. Two basic types of the layered zirconium phosphates are now known and studied in detail. They are a- and g-zirconium phosphates represented by formulae Zr(HPO4)2zH2O (a-ZrP) and Zr(H2PO4)(PO4)z2H2O (g-ZrP), respectively. These compounds were first prepared by Clearfield and Stynes [5] and Clearfield et al. [6] by a reflux method. Both zirconium phosphates have a layered structure. In a-ZrP, metal atoms lie very nearly in a plane and are bridged by monohydrogenphosphate groups, whereas in g-ZrP, zirconium octahedra in a planar sheet are connected by a phosphate tetrahedron and the dihydrogenphosphate groups are linked axially to the metal atoms. Such arrangement of atoms creates sufficiently large openings to the inner “zeolite-type” cavities located between the layers, which enables exchange between protons and outer ions, intercalation of organic molecules, and pillaring. Recently Bortun and coauthors [7] have shown that a-ZrP treatment with NaOH or KOH under mild hydrothermal conditions leads to the formation of a novel metastable zirconium-rich phosphate of the formula Zr2O3(MPO4)znH2O (M 5 Na, K; n 5 1.5–2.0), designated as a c-phase. Characterization of the c-ZrP properties showed that it has a layered structure and exhibits extremely high hydrolytic stability and a preference for large cations uptake. In this paper, we present the results of the synthesis and preliminary characterization of a novel layered zirconium arsenate of the formula Zr2O3(MAsO4)znH2O (M 5 H, Na; n 5 1–3) with the novel c-type layered structure. EXPERIMENTAL Reagents. All reagents (Aldrich) were of analytical grade. Analytical Procedures. A diffractometer (Rigaku model RU 2000) was used with monochromated CuKa radiation (l 5 1.5418 Å). Thermal analysis (DuPont Instruments TA 4000) was performed under nitrogen at a heating rate of 10°C/min. The zirconium, arsenic, and sodium content in the solid was determined with a spectrometer (SpectraSpec DCP-AEC) after dissolving a weighed amount of sample in an HF aqueous solution. IR spectra were obtained on a spectrophotometer (Perkin-Elmer 1720-X FT) by the KBr pellet technique. Electron micrographs were recorded with an electron microscope (Jeol JSM-6100) operating at 20 kV. Synthesis. The layered sodium zirconium arsenate (g-ZrAs–Na) was prepared as follows: 13.35 mL of 70% zirconium propoxide (in propanol) was thoroughly mixed with 40 mL of an aqueous solution containing 2.1 g of As2O5 and 1.8 g of NaOH. This reaction mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed, and treated at 190°C for 10 days without stirring. The resultant solid was recovered by filtration, washed with demineralized water, and dried in air at room temperature. The hydrogen form of zirconium arsenate was prepared by treatment of the sodium form with excess of a 1 M HNO3 aqueous solution. After washing the solid with demineralized water and drying in air at room temperature, c-ZrAs–H was prepared, whereas c-ZrAs–H-70 was obtained after drying the compound at 70°C. n-Alkylamine intercalation compounds were obtained by exposure of c-ZrAs to an atmosphere saturated with amine vapor for 6 days at room temperature. Ion Exchange Study. The exchange of alkali and alkaline earth metal cations on the zirconium arsenate was studied in 0.05 N MCln–M(OH)n (M 5 Li, Na, K, Cs, Ca, Sr, Ba; n 5 1,2) solutions at V:m 5 200:1 (mL:g) at room temperature. The affinity of the exchangers (in hydrogen and salt

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FIG. 1 The XRD powder patterns of (a) c-ZrAs–Na, (b) c-ZrAs–H, and (c) c-ZrAs–H-70.

forms) to some heavy metal cations was studied from solutions containing 1023 M of Mn1 (Mn1 5 Cr31, Mn21, Co21, Ni21, Cu21, Cd21, Hg21, Pb21) and 1 M NaCl (NaNO3) at V:m ratio 200:1 (mL:g). The pH of the solutions after equilibration with the exchanger was measured using a pH meter (Corning, model 340). Initial and final concentrations of alkali and alkaline earth ions in solutions were measured using a atomic absorption spectrometer (Varian SpectrAA-250). The affinity of the exchanger for some transition elements was expressed through the distribution coefficient (Kd, mL/g) values that were found according to the formula Kd 5 [(Co2Ce)/Ce]zV/m, where Co and Ce are the ion concentrations in the initial solution and in the solution after equilibration with adsorbent, respectively, and V/m is the volume-to-mass ratio. RESULTS AND DISCUSSION Under the given experimental conditions c-ZrAs–Na, with relatively low crystallinity, is formed. The X-ray diffraction (XRD) powder pattern of this solid, having a first X-ray reflection at 13.7 Å, is presented in Figure 1a. An increase in time (up to 30 days) or

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FIG. 2 Scanning electron microscopy image of c-ZrAs–H-70. temperature (to 210°C) of thermal treatment, as well as a variation in the alkalinity of the reaction mixture, do not improve significantly the crystallinity of the compound. By treatment of the c-ZrAs–Na with an excess of a 1 M HNO3 and drying the product at room temperature, the protonic form (c-ZrAs–H), with a first reflection at 14.4 Å (Fig. 1b), was prepared. In the case of c-ZrAs–H dried at 70°C, the d spacing of the protonic form decreases to 11.8 Å (Fig. 1c). The scanning electron microscopy photograph of the protonic form of the zirconium arsenate is shown in Figure 2. Its particles have a small size (less then 1 mm) and an irregular form. The lack of a clearly defined morphology correlates with the low crystallinity of the compound. The data of elemental analysis and the proposed formulas for the novel c-type compounds are presented in Table 1. A good agreement between the experimental and calculated values is observed in all cases. The sodium form of the c-zirconium arsenate, prepared hydrothermally, Zr2O3(NaAsO4)z3H2O (c-ZrAs–Na), is converted by topotactic leaching of sodium in acid media into the protonic form, Zr2O3(HAsO4)z3H2O (c-ZrAs–H), which undergoes a partial dehydration when treated thermally at 70°C: Zr2O3(HAsO4)zH2O (c-ZrAs–H-70). The TG curves for the hydrogen forms of the novel zirconium arsenates are presented in Figure 3. In the c-ZrAs–H, the weight loss occurs in three steps. In the first step, which takes place in the temperature range 50 –210°C, a weight loss of 12.8% was found, which we assign to three molecules of interlayer crystal water release (calculated: 12.73%). The weight loss of 2.1% in the second and third steps (T 5 210 – 600°C) is attributed to the release of half a mole of structurally bound water per formula weight, due to the condensation of the hydrogenarsenate groups (calculated: 2.12%). In the case of c-ZrAs–H-70, the total weight TABLE 1 Analytical Data and Experimental Weight Loss (w.l.) at 600°C of the Synthesized Compounds Experimental Compound

Formula

c-ZrAs–Na Zr2O3(NaAsO4)z2H2O c-ZrAs–H Zr2O3(HAsO4)z3H2O c-ZrAs–H-70 Zr2O3(HAsO4)zH2O

Calculated

% Zr % As % Na % w.l. % Zr % As % Na % w.l. 42.2 43.2 46.4

17.6 17.9 19.6

5.3 -

8.4 14.9 6.8

42.59 17.49 42.99 17.65 46.97 19.29

5.37 -

8.40 14.85 6.95

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FIG. 3 TG (solid line) and DTG (dashed line) curves for (a) Zr2O3(HAsO4)z3H2O and (b) Zr2O3(HAsO4)zH2O. loss is much less (6.8%). The thermal decomposition of this compound also occurs in three steps. In the first step, which takes place in the temperature range 50 –190°C, the weight loss is 4.4%; 1 mol of interlayer crystal water is lost (calculated: 4.64%). In the second (190 – 320°C) and the third steps (320 – 600°C), the weight losses are 1.6% and 0.8%, respectively; half a mol of structural water is released (calculated: 2.31%). As seen in Figure 4, the IR spectrum of the c-ZrAs–H-70 has several intense, medium, and

FIG. 4 IR spectrum of c-ZrAs–H-70.

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FIG. 5 The XRD patterns of c-ZrAs–H intercalated with (a) propylamine and (b) n-butylamine. weak bands at 3420, 1622, 994, 909 (sh), 824, 793, 703, 500 (sh), and 427 (m) cm21. The positions and intensities of bands observed are characteristic of the spectra of the polyvalent metal arsenates [8,9]. Considering this, the adsorption bands at 994, 909 (sh), 824, and 793 cm21 could be assigned to symmetric and antisymmetric stretching modes of As–O bonds in the HAsO4 group. The two bands at 500 and 427 cm21 we assign to the vibration of Zr–O bonds of the c-ZrAs matrix or/and to d-(AsO4). A strong intensive band at 3420 cm21 and a medium intensity band at 1622 cm21 could arise from the presence of the free As–OH groups and/or physically bound water. It was found (Fig. 5) that the c-ZrAs–H is able to react easily with propylamine and n-butylamine vapors, giving corresponding amine intercalates with interlayer distances larger than that of the initial material (17.8 Å for c-ZrAs–PrNH2 and 21.5 Å for c-ZrAs–BuNH2). The ability to intercalate amines with expansion of the basal spacings confirms the layered structure of c-ZrAs. The presence in the structure of Zr2O3(HAsO4)z3H2O of the acidic functional group and its layered nature indicates that this compound should behave as a cation exchanger with a theoretical ion exchange capacity (IEC) of 2.36 meq/g. The estimation of the affinity of c-ZrAs–H for alkali and alkaline earth metal ions has been carried out by the potentiometric titration method. The experimental curves showing the dependencies of alkali metal ions uptake by c-ZrAs–H in 0.05 M MCl–MOH (M 5 Li, Na, K, Cs) solutions are presented in Figure 6a. The data show that the exchange of some cations starts at a pH lower than 2, but the values of their uptake remain low until a pH of 4 –5 is reached. In acid solution, c-ZrAs–H preferably exchanges cesium ion and exhibits the following selectivity sequence: Cs1 . K1 . Na1 . Li1. The IEC of the layered zirconium arsenate increases up to 2.0 –2.1 meq/g in the pH range 6.5– 8.0 for K1, Na1, and Li1 cations and to 1.8 meq/g for cesium ions. The maximum alkali metal uptake of 2.2–2.3 meq/g found at pH 11–12 corresponds to the theoretical IEC value. c-ZrAs–H does not release practically any arsenate ions in the

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FIG. 6 Exchange of (a) Li1 (E), Na1 (F), K1 (‚), and Cs1 (Œ) ions, and (b) Ca21 (f), Sr21 (E), and Ba21 (Œ) ions by c-ZrAs–H as a function of pH.

solution in the pH range (2–12) tested, which indicates its extremely high hydrolytic stability, comparable to that of c-zirconium phosphate [7]. Potentiometric titration curves of c-ZrAs–H with alkaline earth metal hydroxides are presented in Figure 6b. As seen, the exchange of alkaline earth metal cations does not depend significantly on the cations. For barium and calcium ions, it starts at pH , 3, gradually increases to 0.5 meq/g at pH 5, then drastically increases up to 2.1–2.3 meq/g with pH increase to 7.0 –7.5, and stays unchanged at higher pH values. The potentiometric curve for stronium is much like that found for barium and calcium ions, except that the strontium uptake begins at higher pH values (pH . 4). The maximum Ba21, Sr21, and Ca21 uptake is 2.3 meq/g (pH 12). The affinity of c-ZrAs (in hydrogen and sodium forms) to some di- and trivalent cations has been determined in pure 1023 M M(NO3)n and 1023 M M(NO3)n 1 1 M NaNO3 solutions (Table 2). The data show that the affinity of c-ZrAs–H to all the ions studied is extremely low (the Kd values are lower than 50). This is in a good agreement with the potentiometric titration data, indicating that this adsorbent has only weak acid functional groups, which are practically nondissociated at pH lower than 3– 4. However, the exchanger in the sodium form exhibits sufficiently high affinity to different polyvalent metal ions. We explain this by the fact that, in this case, the adsorption occurs at a more preferable pH, as well as the lesser affinity for the alkali metals, compared with hydrogen ions. The selectivity sequences found for c-ZrAs–Na are as follows: Pb21 .. Cr31 . Mn21 . Ni21 . Co21 . Hg21 . Cd21 . Cu21 [M(NO3)n solutions], and Pb21 . Cd21 $ Cr31 . Cu21 . Mn21 . Co21 . Ni21 $ Hg21 [M(NO3)n 1 1 M NaNO3 solutions].

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TABLE 2 The Kd Values (in mL/g) and the Equilibrium pH Valuesa for Selected Di- and Trivalent Cation Exchange by the c-Zirconium Arsenates in 1023 M M(NO3)n (A) and 1023 M M(NO3)n 1 1 M NaNO3 (B) Solutions c-ZrAs–H

c-ZrAs–Na

Cation

A

B

A

B

31

46 (2.2) 15 (2.5) ,5 (2.5) ,5 (2.5) 19 (2.5) 10 (2.6) ,5 (2.5) 50 (2.6)

35 (2.2) 410 (2.4) ,5 (2.4) ,5 (2.4) 10 (2.3) 12 (2.4) ,5 (2.3) 45 (2.4)

4100 (7.1) 1690 (7.7) 1100 (7.9) 1360 (7.7) 600 (7.4) 670 (8.1) 780 (7.8) 59000 (7.5)

22400 (6.3) 1200 (6.4) 680 (6.9) 235 (6.2) 14500 (5.6) 23400 (7.7) 260 (6.2) 42500 (6.3)

Cr Mn21 Co21 Ni21 Cu21 Cd21 Hg21 Pb21 a

pH values given in parentheses.

ACKNOWLEDGMENTS We thank the U.S. Department of Energy (grant 198567-A-F1) through the Pacific Northwest National Laboratory under the Efficient Separations and Processing Crosscutting Program of the DOE, Office of Science and Technology, and a CICYT (Spain) research project grant (MAT94 – 0428). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

A. Clearfield, G.H. Nancollas, and R.H. Blessing, in Ion Exchange and Solvent Extraction, ed. J.A. Marinsky and Y. Marcus, Vol. 5, pp. 1–120, Marcel Dekker, New York (1973). Inorganic Ion Exchange Materials, ed. A. Clearfield, CRC Press, Boca Raton, FL (1982). G. Alberti and U. Costantino, in Intercalation Chemistry, ed. M.S. Wittingham and A.J. Jacobson, pp. 147–180, Academic Press, New York (1982). G. Alberti, in Recent Developments in Ion Exchange, ed. P.A. Williams and M.J. Hudson, pp. 233–248, Elsevier, London (1987). A. Clearfield and J.A. Stynes, J. Inorg. Nucl. Chem. 26, 117 (1964). A. Clearfield, R.H. Blessing, and J.A. Stynes, J. Inorg. Nucl. Chem. 30, 2249 (1968). A. Bortun, L. Bortun, and A. Clearfield, Solvent Extr. Ion Exch. 15, 305 (1997). N.G. Chernorukov, I.A. Korshunov, and I.M. Zhuk, Russ. J. Inorg. Chem. 27, 1728 (1982). The Infrared Spectra of Minerals, ed. V.C. Farmer, Mineralogical Society, London (1974).